1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which magnetically excites an atomic nuclear spin of an object to be examined by using an RF signal having a Larmor frequency and reconstructs an image on the basis of a magnetic resonance signal generated by the excitation and a method of setting a capturing condition in the magnetic resonance imaging apparatus. More particularly, the invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method that are capable of performing a non-contrast MRA that obtains an image of a blood flow without using a contrast medium.
2. Description of the Related Art
A MRI (magnetic resonance imaging) method is an imaging method that excites an atomic nuclear spin of an object disposed in a magnetostatic field by using an RF (radio frequency) signal having a Larmor frequency and reconstructs the image on the basis of a MR (magnetic resonance) signal generated by the excitation.
In the field of the magnetic resonance imaging, as a method of obtaining an image of a blood flow, MRA (magnetic resonance angiography) is known. An MRI that does not use a contrast medium is referred to as a non-contrast MRA. As the non-contrast MRA, an FBI (fresh blood imaging) method that performs an ECG (electro cardiogram) synchronization to capture a pumping blood flow ejected from the heart, thereby satisfactorily representing a blood vessel (for example, refer to JP-A No. 2000-5144). The FBI method performs a three dimensional scanning that encodes a frequency in a direction substantially equal to a movement direction of the blood vessel by controlling a gradient magnetic field.
As the non-contrast MRA by the FBI method, a flow-spoiled FBI method in which a difference between the image data captured by changing a delay time of the ECG synchronization is obtained so that an MRA image in which an artery and a vein are distinguished from each other is obtained is disclosed. That is, according to the flow-spoiled FBI method, the difference of the artery signal in a diastole and a systole of the cardiac muscle can be imaged.
Further, in the FBI method, in order to extract a blood flow of the low flow velocity, a flow-dephasing method in which a gradient pulse (Gspoil) is applied in a RO (readout) direction, and a dephase pulse or refocusing pulse is applied to a gradient magnetic field pulse is designed (refer to JP-A-2003-135430). According to the flow-dephasing method, due to the dephase pulse or the refocusing pulse, it is possible to increase the relative signal difference between a signal value from the blood flow of high velocity and a signal value from the blood flow of low velocity. Therefore, it is possible to clearly distinguish the artery and the vein from each other on the basis of the relative signal difference.
That is, in order to distinguish the artery and the vein, it is important to increase the difference between signals in the diastole and the systole. In order to increase the difference between signals in the diastole and the systole, it is need to make an intensity of the signal from the blood flow of high velocity in the systole be small. Therefore, the gradient pulse having a proper intensity in the RO direction is set, and the blood flow signal from the artery in the systole is controlled by the set gradient pulse. In this state, the blood flow signal in the diastole is collected. A difference process or an MIP (maximum intensity projection) process is performed on the blood flow signal collected in the diastole, and only the artery is represented.
Further, a flow preparation scan that performs a pre-scan while changing a parameter such as the intensity of the dephase pulse in the RO direction in the flow-dephasing method is designed (for example, see JP-A-2003-70766). In the flow-preparation scan, it is possible to obtain a suitable parameter by referring the captured image while varying the parameter by using the pre-scan. Furthermore, there is a report concerning the intensity of the dephase pulse in the RO direction (for example, see Miyazaki M, et al., Radiology 227: 890-896, 2003).
According to a study of Norris D G, et al., when capturing using the FSE (fast spin echo) method, echoes belonging to two families, that is, an even echo and an odd echo are generated. In this case, when the intensities of the even echo and the odd echo are A and B, respectively, the signal intensity S0 of the reception signal is represented by Equation (1) (for example, see Norris D G, et al., MRM 27: 142-164, 1992).S02=A2+B2+2AB cos(2θ)   (1)
In this Equation (1), 2θ is a phase difference between the even echo and the odd echo. Further, the even echo belongs to a family that is simultaneously generated with the refocusing of the last spin echo, and the odd echo belongs to a family that is simultaneously generated with the refocusing of the first excited echo.
Further, according to the study of Norris D G, et al., the phase difference 2θ between the even echo and the odd echo is varies due to the influence of the flow velocity, and the phase changed amount 2φ can be represented by Equation (2).2φ=γGv(TE2)   (2)
In this Equation (2), v indicates the flow velocity in the RO direction, TE indicates the distance of the echo strings, G indicates the intensity of the gradient pulse in the RO direction, and γ indicates a coefficient. It can be understood from Equations (1) and (2) that the phase difference 2θ is changed by the phase changed amount φ due to the influence of the flow velocity v of the blood flow, and the signal intensity S0 may have a loss.
The signal difference of the blood flow signal that is used for imaging in the above FBI method is changed corresponding to the flow velocity. Therefore, in the FBI method according to the related art, it is not always true that the maximum signal difference can be obtained between the diastole and the systole. Specifically, in a portion with a low flow velocity, it is difficult to obtain the blood flow signal with high intensity from the artery as the difference between the diastole and the systole.
Further, in the diastole and the systole, the signal from the vein that is considered to have the same flow velocity is deleted by the difference value between the blood flow signals. However, when the flow velocity of the vein is changed in the diastole or the systole, there is a problem in that the signal of the vein is not completely removed from the difference value in the diastole and the systole.